We have had two airlines disappear this year, Malaysia Airlines Flight 370 in March and another this month. Both hit the water and sank. The most recent incident at least left debris on the surface that marked the crash site, but even so it took a substantial search to locate the wreckage. MH370 resulted in a huge search effort that was to my knowledge fruitless.
Our friend, Ken White had a very reasonable question, Why don’t airliners have hydrostatically released EPIRBs? Good question. Having two would not be unreasonable. The devices might also be activated by sudden high g-forces.